Liquid crystals consisting of mixed cholesteric esters are known to undergo color changes at very specific temperatures in the neighborhood of normal room temperature or body temperature. Consequently, such liquid crystals have been found to be very sensitive indicators of small differences in temperature. This has permitted the liquid crystal materials to be used as extremely accurate temperature indicators and to be used to monitor locations in which temperature differences exist, by providing visual indication of the location of zones of differing heat capacity or differing vascularity in living tissues, for medical or veterinarial purposes, or the location of discontinuities, such as flaws in articles supplied with heat for industrial or constructional purposes.
The practical use of liquid crystals has been somewhat inconvenient in that successful surface contact with contoured surfaces has not been possible with the normally available liquid crystal materials coated onto a substrate, such as a Mylar substrate.
An object of this invention is to provide a device containing liquid crystals, such as microencapsulated liquid crystals, which is capable of providing thermal maps or thermal images of non-planar or planar surfaces. Uses for this invention may be divided into at least two large classes.
1. Thermal maps or thermal images of contoured or planar surfaces of the human body.
2. Thermal maps or thermal images of contoured or planar surfaces of industrial or structural items.
Since liquid crystals have the ability to selectively scatter light and produce color images dependent upon the temperature of their environment, they may be used to project a visual, color picture of the transient temperature anomalies, or minute thermal gradients associated with material discontinuities. Locations of zones or regions of differing temperatures in the human body may be observable in a color picture. As for example, infection can cause inflamation, increased blood flow and heat in an internal organ. That heat may be projected to the surface of the skin and observable in a thermograph. An inflamed appendix or gall bladder infection often creates an area of elevated temperature on the skin. Diseases of the veins and arteries which cause clotting, dilation, or narrowing of the vessels similarly may cause temperature variations in adjacent areas of the skin. These temperature variations may be lower or higher than the temperature of surrounding tissues.
In industrial or structural items discontinuities may, for example, be debonds, cracks, or other defect areas, which sufficiently impede the flow of heat to disturb the normal temperature patterns of a material being tested. The defects then appear as distinct color patterns, as a result of their impaired thermal transmission characteristics.
Until recently, the only thermography available was performed with very expensive equipment and required a controlled temperature environment and a draft free room. The invention herein provides for the first time a low cost, portable instrument for thermographic use in detecting or determining the temperature of a planar or contoured surface.
The preferred thermographic compositions of this invention comprise mixtures of cholesteryl esters as the core material of microcapsules having transparent or translucent walls. Preferably, the microcapsule wall material is polymeric film material such as gelatin urea-formaldehyde, or melamine-formaldehyde polymer material. When the microcapsule wall material is gelatin, the method of encapsulation disclosed in U.S. Pat. No. 2,800,456 is used. For microcapsules having urea-formaldehyde polymer wall material, the encapsulation method of U.S. Pat. No. 4,001,140, particularly, Example 1 thereof, is advantageously used. For microcapsules having melamine-formaldehyde polymer wall material, the encapsulation method of U.S. Pat. No. 4,100,103 is advantageously used.
The thermographic core material of the microcapsules used in the composition of this invention is a mixture of cholesteryl pelargonate, oleyl cholesteryl carbonate, cholesteryl propionate and cholesteryl chloride. The point at which a readily-observable temperature-dependent color change in the composition occurs can be varied by varying the proportion of the four cholesteryl derivatives in the core material mixture. In all cases, the compositions contain major amounts of cholesteryl pelargonate and oleyl cholesteryl carbonate, and minor amounts of cholesteryl propionate and cholesteryl chloride.
The compositions of this invention give a regular series of more-or-less evenly-spaced color changes of gray to red to green to blue over a total range of 3 to 4 centigrade degrees for use in measuring surface temperature of the human body, normal and pathological. Compositions showing gray-red-green-blue color changes in the range of about 27.degree. C. to about 37.degree. C. are useful. A series of compositions can be prepared, varying in one centigrade degree increments (or less if desired) from a 30.degree. C. composition to a 37.degree. C. composition. Particularly useful compositions of this type can be made by the microencapsulation of core materials of the following formulations:
TABLE I ______________________________________ Oleyl Pro- Middle-Green Pelargonate Carbonate pionate Chloride Temperature (%) (%) (%) (%) ______________________________________ (1) 30.degree. 66.4 21.6 5.5 6.5 (2) 31.degree. 66.5 22.0 4.6 6.9 (3) 32.degree. 68.0 20.3 4.7 7.0 (4) 33.degree. 70.0 18.3 4.7 7.0 (5) 34.degree. 71.0 17.3 4.7 7.0 (6) 35.degree. 72.7 15.6 4.7 7.0 (7) 36.degree. 73.2 15.0 5.4 6.4 (8) 37.degree. 73.4 14.5 5.6 6.5 ______________________________________
A special feature of this core material is directed to compositions comprising preferably about equal parts of two of the microencapsulated core material formulations, chosen with a 3.degree. increment, so as to give over-lapping ranges with a total color transition width of about 7 centigrade degrees. However, satisfactory compositions have been obtained with unequal parts up to a ratio of 70-30, without significant loss of brightness. Thus, one part of microencapsulated core material of the 30.degree. composition, No. 1, mixed with one part of microencapsulated core material of the 33.degree. composition, No. 4, gives a composition showing regular color changes, in response to temperature changes from about 27.5.degree. C. to about 34.degree. C. The partial over-lapping of the two color transition ranges of the 30.degree. C. composition and the 33.degree. C. composition gives rise to seven readily-distinguishable color changes: alpha gray, beta red, beta green, beta blue, gamma red, gamma green and gamma blue. The beta hues are predominantly derived from the lower-range composition, namely the 30.degree. C. composition in this case, and the gamma hues are predominantly derived from the higher-range composition, that is the 33.degree. C. composition in this case. The alpha gray is the gray of the 30.degree. C. composition. The beta hues are the primary hues of the 30.degree. C. composition, shaded by the appearance of the gray and red of the 33.degree. C. composition. The gamma hues are the primary hues of the 33.degree. C., shaded by the blue of the 30.degree. C. composition. The beta hues are readily distinguished from the gamma hues such that the entire color transition range gives seven readily recognized colors in response to temperature variations over about a 7-degree range.
Similarly useful mixtures, exhibiting a 7-degree color transition range, but operating at higher temperatures can be made with equal parts of the following compositions: (from the above formulated table) Nos. (2) and (5), Nos. (3) and (6), Nos. (4) and (7), and Nos. (5) and (8).
For greater sensitivity to small temperature changes, compositions having a narrower color transition range are made. Compositions in this formulation range have a color transition range width of about 2 centigrade degrees for the red to green to blue transition, or about 3 centigrade degrees for the total gray through blue transition. A series of seven compositions can be prepared, varying in one-degree increments (or less, if desired) from about 24.degree. C. to 35.degree. C. as shown in the formulation table set forth.
TABLE II ______________________________________ Oleyl Pro- Middle-Green Pelargonate Carbonate pionate Chloride Temperature (%) (%) (%) (%) ______________________________________ (9) 24.degree. 58.6 31.7 4.1 5.6 (10) 26.degree. 61.1 29.6 3.7 5.6 (11) 28.degree. 63.0 28.4 3.0 5.6 (12) 29.degree. 58.5 33.9 2.4 5.2 (13) 30.degree. 60.7 31.5 2.4 5.4 (14) 31.degree. 61.2 31.5 2.5 4.8 (15) 32.degree. 62.2 30.7 2.1 5.0 (16) 33.degree. 63.5 29.2 2.2 5.1 (17) 34.degree. 65.3 27.2 2.6 4.9 (18) 35.degree. 66.3 26.2 2.6 4.9 ______________________________________
Mixing of equal parts of related formulations, after separate microencapsulation thereof, having over-lapping color transition ranges can be effected to give compositions having a beta red to gamma blue transition width of about 3.5 centigrade degrees, or a width of about 4 centigrade degrees, for all of the alpha gray-gamma blue transitions. From the formulation table set forth above, particularly useful compositions are obtained by mixing formulations Nos. 9 and 10, Nos. 10 and 11, Nos. 11 and 13, Nos. 12 and 14, Nos. 13 and 15, Nos. 14 and 16, Nos. 15 and 17, and Nos. 16 and 18.
The various formulations set out in the table above and variations thereon which the artisan can readily derive from the teachings of this disclosure are useful as visual temperature indicators when applied to a surface such as the human skin. The formulations are advantageously used as the core material in microcapsules, having substantially transparent or translucent polymeric wall material. The microcapsule containing one of the thermographic formulations, either separately or mixed with microcapsules containing a second of the thermographic formulations, are most useful when coated onto a substrate sheet material.
Among the more useful flexible binders for holding the microcapsules on the substrate are polyurethane latexes, such as those sold under the trademarks "A-2701-44" by Hughson Chemical, Erie, PA, "Desmocoll E-471" and "E-723" by Mobay Chemical Corporation, Pittsburg, PA, and "Hooker 2050-L", "2030" and "2060" by Hooker Chemical Corporation, Hicksville, NY. The preferred flexible binder latex is "Desmocoll E-723".
In practice, in a device of this invention, a substrate, preferably a non-transparent film, particularly a polyurethane film containing carbon black is coated with the described microcapsules, each of which contains, as core material, a micro-droplet of a cholesteric formulation, as set out in the table above. It is obvious that any flexible, elastic material which is compatible with the encapsulated cholesteric material and the fill material can be used as the substrate. A microcapsular coating slurry specifically for flexibility and elasticity is prepared by mixing:
121 grams aqueous microcapsule slurry (56.1% solids) PA1 34 grams of 40% polyurethane latex (Desmocoll E-732) PA1 6 grams of distilled water PA1 2 grams of a 1% aqueous solution of alkylaryl polyether alcohol wetting agent, (Triton X-100, Rohm and Haas, Philadelphia, PA).
Throughout this disclosure, percents are percents by weight, and temperature is expressed in degrees centigrade.